Electron-correlation effects in the photoionization of ${\mathrm{N}}_2$

We have implemented a multichannel configuration-interaction complete-active-space (MCCI CAS) approximation to study electron-correlation effects in molecular photoionization. This approach includes both target relaxation and correlation due to coupling between different asymptotic scattering channels. The method employs a complete-active-space configuration-interaction wave function to represent the bound target states where the configuration interaction is computed using a single-center expansion to evaluate all integrals. The scattering equations are then solved using the Schwinger variational method. We present results of a detailed MCCI-CAS-Schwinger study of the photoionization of molecular nitrogen in the photon energy region of 19–26 eV, including nine coupled electronic channels. The results are in good agreement with the available experimental data. In particular, we obtain theoretical cross sections including interactions with the ionization channel leading to the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ state of ${\mathrm{N}}_2^{+}$. The theoretical results are found to have a resonance structure similar to that observed in the experimental photoelectron asymmetry parameters in the (3${\mathrm{\sigma }}_{\mathit{g}}$${)}^{\mathrm{-}1}$ ionization channel near 22 eV.